Double-ended transition state (TS) search methods guess TS structures via input reactant and product structures.
Double-ended TS searches start from reactants and products to generate a guess for the transition state connecting these structures. Currently, the Rowan platform supports the freezing-string method (FSM) for double-ended TS searches. FSM starts from reactant and product geometries and forms "frozen" strings of geometries that expand towards the opposite string end while attempting to stay within the reaction channel. Once the ends are connected, the peak along the path is chosen as a guess for the transition state (TS) and optimized (learn more about double ended string methods in our Guessing Transition States blog).
To perform a double-ended TS search, input the reactant and product structures for the reaction of interest. If multiple reactants or products are involved, place all of them in the same 3D input. If you have prior knowledge of the reaction mechanism, it can be beneficial to arrange the reactant and product molecules in the reaction channel to accelerate the process, but it is not necessary. Atom numbering should be consistent between reactants and products, otherwise the method may cause significant rearrangements and an incorrect TS will result.
Double-ended TS search can start from unoptimized structures, but we recommend optimizing the starting structures with Optimize Inputs unless structures have been carefully placed in the reaction channel. (Placing structures manually can sometimes necessary for reactions that don't form van der Waals complexes or that optimize to structures far from the TS). Once the double-ended TS search is complete, the highest energy structure along the path is usually a good guess for the transition state; if Optimize TS is selected, a TS optimization will be performed on this structure.